1. Field of the Invention
The present invention relates to a method for producing gamma ferric hydroxyoxide by means of at least two oxidation steps differing in intensity, to produce a raw material for magnetic powders used to make magnetic recording media.
2. Description of the Prior Art
When a magnetic powder such as gamma ferric oxide is prepared, there can be numerous starting materials including acicular alpha ferric hydroxyoxide (geothite) or acicular gamma ferric hydroxyoxide (lepidocrocite). These materials are dehydrated, reduced and then oxidized to form the desired gamma ferric oxide. In this particular case, the gamma form has an advantage compared with the alpha form in that it has superior dispersion characteristics because it produces crystals with no branches and it can produce a magnetic powder having a high residual magnetic flux density. However, the use of gamma ferric hydroxyoxide as a starting material requires severe conditions of formation as compared with the alpha form so that a method of manufacturing magnetic powders utilizing the gamma form as the starting material is not widely employed.
The lepidocrocite is produced by generating ferrous hydroxide by the reaction of an alkali metal hydroxide such as sodium hydroxide with a ferrous salt such as ferrous chloride in aqueous alkaline solution by passing an oxidizing gas into the aqueous alkaline solution. It is known that the gamma ferric hydroxyoxide is produced through an intermediate product which is generated by the process known as green rust.
Upon the production of seed crystals, the usual technique would be to grow crystals from such seeds to a desired size. Consequently, it is preferable that the seed crystal be as small as possible so that the ultimate crystal size can be small. However, when lepidocrocite is formed, it is not usually possible to provide a suitably small sized seed crystal if high speed oxidation is carried out, as the crystal under these conditions becomes vein-like in shape and appearance and is not entirely suitable for transformation into ferric oxide usable in magnetic recording media.